Rolling plants to produce thin sheet are well known and widely used in the state of the art.
Normally, in such plants to produce strip having thicknesses of the order of about one millimeter (mm.), a casting leaving a normal continuous casting plant with a thickness of about 200 to 250 mm. is sheared into slabs having a length of about 12 to 15 meters (m.).
These slabs generally undergo a post-heating step within a temperature equalization furnace and are then subjected to a process of reduction of width, for instance, by means of vertical rolling rolls.
Thereafter, after having been advantageously subjected to one or more processes to remove scale which forms during the thermal treatments, the slabs come into cooperation with at least one reversible rolling stand.
The rolling cycle normally used, as detailed in U.S. Pat. Nos. 4,430,874 and 4,503,697 for instance, subjects these slabs to a plurality of successive passes alternately in one direction and the reverse direction within one or two reversible rolling stands to form a transfer bar or a strip having a thickness of about 25 to 30 mm.
The bar or strip thus obtained is sent lastly to a continuous, semi-continuous, or reversing finishing mill which reduces the thickness of the strip to the required value.
The finished strip downstream of this finishing mill is wound in coils on a winding machine.
The plants of the state of the art entail a series of drawbacks. First of all, the number of rolling passes is generally small, and this leads to a great reduction of thickness in each pass and therefore to an increase in energy consumption per unit of rolled steel.
Moreover, in such plants the finishing stands positioned downstream are not used during the rolling carried out by the upstream reversible rolling stands.
It follows that at the time of their use the rolls of the downstream stands have temperatures substantially different from the temperatures of the rolls of the upstream stands, and this creates problems as regards the flatness and profile of the strip.
Lastly, to be able to carry out this method, the rolling plants commonly used require great lengths for their rolling line upstream and downstream of the reversible rolling stand. In fact, the progressive reductions of the thickness of the slab, transfer bar and strip lead to a corresponding increase in the length of the slab, transfer bar and strip requiring a plant large enough to handle the rather long strip.
Obviously, the whole length of the transfer bar has to proceed outside the reversible rolling stand before its direction of feed is reversed and the transfer bar undergoes a further pass through the reversible rolling stand.
For this reason and given the dimensional values previously mentioned for the strip, it is necessary to provide a free space of at least 50 to 70 m. between the temperature equalization furnace and the rolling stand and a free space of at least 80 to 120 m. between the rolling stand and the finishing mill.
It is clear that these dimensions entail a considerable engagement of resources in terms of the sizes of the factory and in terms of energy costs and costs for the setting-up and upkeep of the plants.
The article by Vladimir B. Ginzburg and Winfried F. Schmiedberg in the trade journal Iron and Steel Engineer of April 1986 discloses a method in which a plurality of rolling stands is used in one direction and the opposite direction until the final thickness of the strip is achieved, but there is no disclosure of how to obviate too great wear of the rolls of the downstream stands and to reduce losses of heat.
The rolling plants of the state of the art are also problematic in that they may receive slabs from two or more continuous casters where the slabs produced have generally equal thicknesses. By continuously casting slabs of generally equal thickness, the rolling plant is limited to producing only those grades of steel which can be produced from that particular slab thickness. Further, rolling plants which receive single thickness slabs from a continuous caster are usually limited to rolling either plate or strip. Thus, a need has arisen for a rolling plant which includes at least two continuous casters which produce slabs of different thicknesses and a rolling mill which can roll different thickness slabs to increase the number of grades of steel the plant can produce and with a minimum delay between slab processing.
In the conventional reversing rolling plants, thermomechanical treatment during rolling is conducted by either conventional hot rolling, controlled rolling, low finishing temperature rolling or continuum rolling. In conventional hot rolling, the hot rolling of steel is conducted continuously and is usually finished above the upper cooling transformation temperature Ar.sub.3. The upper cooling transformation temperature Ar.sub.3 is the temperature at which the austenite (the gamma phase) in the steel begins to transform into ferrite such that there is a mixture of austenite and ferrite in the steel (the gamma-alpha two-phase mixture). The exact temperature that the transformation occurs depends upon the content of the carbon in the steel, but usually is in the range of about 720 to about 920 degrees C. The temperature range above the upper cooling transformation temperature Ar.sub.3 is referred to herein as the "gamma region".
In controlled rolling, the rolling metal is interrupted by one or two delays which allows one to deform the steel first in the gamma-region and then in a temperature range between the upper cooling transformation temperature Ar.sub.3 and the lower cooling transformation temperature Ar.sub.1 (the gamma-alpha two-phase region). At the lower cooling transfer temperature, all of the austenite has transformed into ferrite, such that there is only ferrite in the steel. Again, the exact temperature that the transformation occurs depends upon the content of the carbon in the steel, but usually is about 720 degrees C.
In low finishing temperature rolling, the finishing rolling passes are conducted in a temperature range between room temperature and below the lower cooling transformation temperature Ar.sub.1 (the alpha region), usually in the range of about 600 to about 720 degrees C. (the upper end of the alpha region). When the temperature of the steel enters the alpha region the ferrite begins to transform into pearlite.
A rolling plant is capable of continuum rolling when it is able to achieve deformation in the gamma-region, gamma-alpha region and alpha region. Conventional reversing rolling hot strip mills have not been able to produce thin gauges by rolling in the lower end of the alpha region at a temperature range of about 20 to about 400 degrees C., because the transfer bar that enters the finishing mill stands is usually too thick, ranging from 20 to 30 mm; so it would require either to substantially increase mill roll power or to increase the number of mill roll stands or both to roll thin gauges at low temperatures. The prior art solution to this problem has been to ship the product to a cold rolling plant for final processing. This obviously increases the cost of the final product and increases the manufacturing time.
Hence, a need has arisen for a reversing rolling plant which can achieve rolling in the gamma region, gamma-alpha region, and both the upper and lower ends of the alpha region. That is, there is a need for a reversing hot rolling plant which can produce intermediate thin strip of about 8 mm to about 4 mm in coil form to conserve heat and to roll this strip down to finish gauge while maintaining precise control of mechanical deformation by controlling reduction in thickness, mill speed, and cooling rate of the strip; thus allowing the plant to roll a relatively greater number of different steel products which are presently produced by cold mills. The present invention satisfies this need.
The present inventors have studied, tested, and created and developed this invention to overcome the shortcomings of the state of the art and to achieve further advantages which will be apparent after reviewing the foregoing and following specification.